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Modern wireless digital communication began in the Hawaiian Islands,
where large chunks of Pacific Ocean separated the users and the telephone
system was inadequate.
2.3.1. The Electromagnetic Spectrum
When electrons move, they create electromagnetic waves that can propagate through free space. The number of oscillation per second of an electromagnetic wave is called its frequency, f, and measured in hertz (Hz). The distance of two consecutive maxima is called wavelength and universally designated by l (lambda).
By attaching an antenna of the appropriate size to an electrical circuit, the electromagnetic waves can be broadcasted efficiently and received by a receiver some distance away. All wireless communication is based on this principle.
In vacuum, all electromagnetic waves travel at the same speed, usually called the speed of light, c, approximately 3 x 108 m/sec. In copper or fiber the speed slows to about 2/3 of this value and becomes slightly frequency dependent.
The fundamental relation between f, l, and c (in vacuum) is
lf = c
For example: 1-MHz waves are about 300 m long and 1-cm waves have a
frequency of 30 GHz.
Fig. 2-11. The electromagnetic spectrum and its uses for communication.
The electromagnetic spectrum is shown in Fig. 2-11. The radio, microwave, infrared, and visible light portions of the spectrum can all be used for transmitting information by modulating the amplitude, frequency, or phase of the wave. Ultraviolet light, X-rays, and gamma rays would be even better, due to their higher frequencies, but they are hard to produce and modulate, do not propagate well through buildings, and are dangerous to living things.
LF, MF, ... are official ITU (International Telecommunication Union) names and are based on wavelengths.
The amount of information that an electromagnetic wave can carry is related to its bandwidth. With current technology, it is possible to encode a few bits per Hertz at low frequencies, but often as many as 40 under certain conditions at high frequencies, so a cable with 500 MHz bandwidth can carry several gigabits/sec.
There are national and international agreement about who gets to use which frequencies. World-wide, it is an agency of ITU-R (WARC), in US the work is done by FCC (Federal Communication Commission).
Most transmissions use a narrow frequency band ((f/f<<1)to get the best
reception (many watts/Hz). However, there are some exception from this
rule (i.e. spread spectrum popular in military communications).
2.3.2. Radio Transmission
Radio Waves are easy to generate, can travel long distances, and penetrate building easily, so they are widely used for communications, both indoors and outdoors. They are also omnidirectional, so the transmitter and receiver do not have to be aligned physically. This feature is sometimes good, but sometimes bad.
The properties of radio waves are frequency dependent. At low frequencies they pass through obstacles well, but the power falls off sharply with distance from the source. At high frequencies, radio waves tend to travel in straight lines and bounce off obstacles. They are also absorbed by rain. At all frequencies, they are subject to interference from motors and other electrical equipment.
Due to radio's ability to travel long distances, interference between users is a problem. For this reasons, all governments license the use user of radio transmitters.
Fig. 2-12. (a) In the VLF, VF, and MF bands, radio waves follow
the curvature of the earth.
(b) In the HF they bounce off the
ionosphere.
In the VLF, LF, and MF bands, radio waves follow the ground (Fig. 2-12(a)) and can be detected for about 1000 km at the lower frequencies, less at the higher ones. The main problem with using these bands for data communication is relatively low bandwidth they offer.
In the HF and VHF bands, the ground waves tend to be absorbed by the earth,
but if they reach the ionosphere (a layer of charged particles circling the
earth at a height of 100 to 500 km) are refracted (Fig. 2-12(b)) by it and
sent back to earth. Amateur radio operators use these bands to talk
long distance.
2.3.3. Microwave Transmission
Above 100 MHz, the waves travel in straight lines and can therefore be narrowly focused. Concentrating all the energy into a small beam using parabolic antenna gives a much higher signal to noise ratio, but the transmitting and receiving antennas must be accurately aligned with each other.
Before fibre optics, for decades, these microwaves formed the heart of the long-distance telephone transmission system.
Microwaves do not pass through buildings well. In addition, even though the beam is well focused, there is still some divergence in space. Some waves may be refracted off low lying atmospheric layers and may take slightly longer to arrive than direct waves. Being out of phase they can cancel the signal. This effect is called multipath fading and is often a serious problem. It is weather and frequency dependent.
Bands up to 10 GHz are now in routine use, but at about 8 GHz a new problem sets in: absorption by water (rain). The only solution is to shut off links that are being rained on and route around them.
Microwave is also relatively inexpensive. Putting up two simple towers (maybe just big poles with four guy wires) and putting antennas on each one may be cheaper than burying 50 km of fibre through a congested urban area, and it may also be cheaper than leasing the telephone company fibre.
Microwaves have also another important use. We are speaking about cordless
telephones, garage door openers, wireless hi-fi speakers, security gates
etc. These devices use so called Industrial/Scientific/Medical bands
forming an exception to the licensing rule: transmitters using these
bands do not require government licensing. One band is allocated
world-wide: 2.400-2.484 GHz. These bands are popular also for various
forms of short-range wireless networking.
2.3.4. Infrared and Millimeter Waves
Unguided infrared and millimeter waves are widely used for short-range communication (remote control of televisions and stereos). They are relatively directional, cheap and easy to build, but they do not pass through the solid objects. For this reason, no government license is needed to operate an infrared system.
These properties have made infrared an interesting candidate for indoor wireless LANs (i.e. portable computers with infrared capability can be on local LAN without having to physically connect to it.
Infrared communication cannot be used outdoors because the sun shines
as brightly in the infrared as in visible spectrum.
2.3.5. Lightwave Transmission
Unguided optical signaling has been in use for centuries.
A modern application is to connect the LANs in two building via lasers mounted on their rooftops. Optical signaling using lasers is unidirectional, so each building needs its own laser and its own photodetector. This scheme offers very high bandwidth and very low cost. It is also relatively easy to install and does not require license.
The laser's strength, a very narrow beam, is also a weakness here. Aiming a laser beam 1 mm wide at a target 1 mm wide 500 m away could be a problem. Usually, lenses are put into the system to defocus the beam slightly.
A disadvantage is that laser beams cannot penetrate rain or thick fog. Some other phenomena in the atmosphere can also influence the communication using laser (Fig. 2-13.).
Fig. 2-13. Convection currents can interfere with laser
communication systems.
A bidirectional system, with two
lasers, is pictured here.